Cumulative Impact Assessment under the National Environmental Policy Act: An Analysis of Recent Case Law

The assessment of cumulative impacts is one of the most difficult tasks a NEPA (National Environmental Policy Act) practitioner faces when preparing an Environmental Assessment or Environmental Impact Statement, and it recently has become an increasing focus area of legal challenges. Historically, federal agencies have a very poor track record in this litigation. This research examines 25 recent judicial opinions involving challenges to various NEPA documents' cumulative impact analyses, from the federal Ninth Circuit Court of Appeals. Challengers were victorious on their claims of inadequate analysis in 60% of the cases decided in the 10-year analysis period. In recent years, the success rate for challengers has risen even higher, to victories in eight of 11 cases (72%). The main reasons federal agencies have lost these court challenges is because they failed to present any cumulative impact analysis whatsoever in their NEPA document; they left out obvious past, present, or reasonably foreseeable future actions; or they provided undocumented assertions that their projects would not cause any cumulative impacts. Four key lessons for practitioners emerge from the results: (1) Make sure to include a discussion of cumulative impacts for each resource and include all past, present, and reasonably foreseeable future actions; (2) Do not make unsubstantiated claims about cumulative impacts; (3) Assessments do not have to be perfect but should be thorough and based on data and explained rationales; and (4) Do not tie assessments solely to programmatic or non-NEPA documents.